Abstract
The quality of rice seed is very important for rice production. In the process of rice seed production, seed drying is an important link in the whole process of seed quality control, and the problems in mechanical drying are important factors affecting the quality of rice seed. It is easy to encounter rain weather when the sterile seeds are dried by natural air, which seriously affects the seed vigor. High-quality rice seed is an important guarantee to achieve high yield of rice, and seed germination rate is one of the factors directly affecting rice yield. The drying process has the greatest effect on the quality and germination rate of sterile rice seeds. Selecting reasonable drying technology is an important means to ensure the germination rate of sterile rice seeds. In this paper, the dewatering and drying technology of rice sterile line seeds in intelligent baking room was studied. An adaptive integral sliding mode control algorithm based on Smith prediction was proposed for intelligent baking room temperature. The lag in the system was compensated by Smith prediction; the uncertainty of the system model and the complex interference under variable operating conditions were overcome by the robustness of sliding mode control. And the chattering of the sliding mode was eliminated by the adaptive integral sliding mode surface. The seeds of three rice varieties were dried, the drying dehydration rate, seed germination rate, and seed vigor were measured, and the changes of seed moisture and temperature during drying were observed. The results showed that there was little difference in the seed vigor after drying and air dried, which proved that the dryer could be used to dry different kinds of male sterile seeds. Compared with air dried seeds, the germination rate and germination potential of all mechanically dried seeds have little difference, and the overall difference of germination index and vitality index is also small. This study solved the problem that the seeds of male sterile lines could not be dehydrated and dried in time of rainy weather, which is of great significance to improve seed quality and economic benefits.
1. Introduction
The quality of rice seeds is of great significance to the yield of rice [1]. At present, there are many researches on grain drying in China, but there are few researches on rice seed drying technology [2]. There is a significant difference in dryness between seeds and rice grains [3]. Rice seeds are mainly used for reproduction, so attention should be paid to maintaining their active quality in seed drying. While grain is mainly used for food, we should pay attention to grain drying to ensure taste and nutrition. Rice seeds are harvested earlier than normal rice grains. The moisture content of rice seeds at harvest time is relatively high, so the moisture content of rice seeds must be reduced to 13% in time. If rice seeds are not dried in time, they will lose breeding value due to fever and deterioration, resulting in unnecessary losses [4]. The mechanical drying rate of rice seeds in China in 2019 was about 30 percent. The mechanical drying technology of rice seeds in China still has great development and research space.
Compared with rice grains, rice seeds have special technological requirements [5]. High drying temperature can easily destroy the internal structure of seeds and make seeds lose their breeding value. Many scholars at home and abroad have studied the optimal drying temperature of rice seeds [6]. Huang et al. [7] found that the drying temperature at the highest seed vigor index was 45°C. With the increase of drying temperature, the germination rate of seeds will significantly decrease, the conductivity of seeds will increase, and the internal structure of seeds will be damaged with the increase of temperature. The study of OYOH and MENKITI [8] showed that 40°C was the optimal temperature for drying rice seeds, which could better guarantee the seed activity. Cortés-Rojas et al. [9] reported that the germination rate of rice seeds would always decrease after the drying temperature exceeded 40°C. Liu et al. [10] found that the temperature of seeds should be monitored in real time during drying. The seed germination rate decreased significantly when the grain temperature exceeded 34°C, and the seed temperature was about 34°C when the drying temperature was 40°C. Therefore, the drying temperature of rice seeds should not exceed 40°C, so as to effectively guarantee the germination rate of rice seeds.
In addition to the research on drying temperature, many experts have studied other technical parameters of rice seed drying [11]. Amini et al. [12] found in their study that the most important factors affecting the seed drying quality were drying temperature and drying speed. When de Faria et al. [13] dried corn seeds, they found that seeds with higher water content were more vulnerable to heat loss during drying. They suggest staged drying, starting with drying high-moisture seeds at low temperatures, increasing the drying temperature to 46°C when the seed moisture content is about 20%, and then subsequent drying. Research by Zhao et al. [14] showed that, in the process of drying seeds, applying vibration of a certain frequency to seeds can accelerate the loss of water inside seeds and further achieve high-quality drying effect.
As the drying time of rice seeds increases, the moisture difference between the inside and outside of rice seeds increases continuously, which will cause cracks in the seeds after stress and affect the quality of seeds. Therefore, retarding treatment should be added in the process of seed drying [15]. After drying for a period of time, slow seed treatment can make the internal water of the seed to the outer layer. Thus, the water content of the inner and outer layers of seeds is uniform, the difference of water content is reduced, and the possibility of crack is reduced. Relevant studies [16, 17] show that retarding treatment is of great help to improve drying efficiency and drying quality. Bakhtavar and Afzal [18] showed that high temperature drying for a long time would reduce the vigor of seeds. After drying for a period of time, the seed vigor would not decrease after 5–20 min retarding treatment. Retarding treatment can reduce the stress cracks in the drying process of seeds and can avoid seeds in high temperature and dry environment for a long time, reducing the heat damage of seeds.
At present, there are many researches on mechanical dewatering and drying of rice seeds, but there are few researches on mechanical dewatering and drying of sterile line seeds. Temperature control is the key factor to ensure food quality during drying of sterile line seeds. Due to the characteristics of time-varying, strong coupling, and nonlinearity, the traditional PID control is difficult to meet the requirements of process control. An adaptive integral sliding mode control algorithm based on Smith prediction is proposed in this paper. The application of the algorithm can not only achieve the expected drying effect of sterile line seeds, but also enhance the seed vigor, improve the seed quality, and promote the large-scale production of seeds.
The innovations of this paper are as follows:(1)Use Smith estimation to compensate for the lag in the system.(2)The robustness of sliding mode control is used to overcome the uncertainty of system model and complex interference under variable working conditions.(3)Adaptive integral sliding mode surface is used to eliminate chattering.
This paper consists of five main parts: the first part is the introduction, the second part is state of the art, the third part is methodology, the fourth part is result analysis and discussion, and the fifth part is the conclusion.
2. State of the Art
2.1. Grain Drying Machinery and Characteristics
2.1.1. Main Grain Drying Machinery, Drying Mode, and Drying Effect
At present, China’s grain drying machinery used for drying sterile seeds mainly includes horizontal circulating vertical dryers, mixed circulating vertical dryers, static horizontal dryers, improved smoke oven, and mixed flowing static chamber dryers [19]. According to the drying principle and mechanical structure, it can be divided into intermittent drying and continuous drying modes [20]. The horizontal circulation dryer and the mixed circulation vertical dryer are set to intermittent drying mode; the static horizontal dryer and mixed flow static dryer are set to the continuous drying mode.
After years of testing and production practice, no matter what kind of drying machine is used, under the condition of proper drying temperature and normal seed vigor, the seed germination rate and vigor after mechanical drying can reach the level of air dried seed. However, different kinds of drying machinery have great differences in drying dehydration rate, seed temperature and moisture variation, damage rate, and drying cost.
2.1.2. Drying Characteristics of Various Drying Machineries
After years of testing and research, the technical parameters of various drying machinery drying sterile line seeds are shown in Table 1. It can be seen from Table 1 that there are great differences in the technical parameters of the four drying machines. First of all, drying temperature refers to the wind temperature due to the mechanical structure of a variety of dryer differences. Static horizontal dryer is (40 ± 2) °C. Horizontal circulation vertical dryer is (42 ± 2) °C. Mixed circulation vertical dryer is (43 ± 2) °C. Mixed flow static chamber dryer is (55 ± 2) °C. Second, the capacity of each dryer (the amount of seed after drying) is different; the largest is the circulation vertical dryer at full load, about 8.0 t. The least is the static horizontal dryer, about 3.5 T. Third, the dehydration rate of each drying machine is very different. Mixed flow static chamber dryer was the highest, up to (0.9 ± 0.1) %/h. Vertical dryers with cross flow circulation were the lowest, only (0.3 ± 0.05) %/h. Mixed circulation vertical dryer is (0.55 ± 0.05) %/h; static horizontal dryer is (0.8 ± 0.1) %/h. Fourth, the seed damage rate of different models is also large. Because the lower part of the vertical dryer is conveying seeds, and the number of seed cycles is many, the seed damage rate is as high as 3.87%. Especially the seeds with high rate of cracked glumes and higher seed damage rate are not suitable for mechanical drying of sterile seeds. The seed damage rate of static horizontal dryer and mixed flow static chamber dryer is less than 1%, which is suitable for mechanical drying of sterile seeds. Fifth, there is difference of drying cost. The drying cost of cross flow circulating vertical dryer and static horizontal dryer using fuel hot blast stove is 0.25–0.40 yuan/kg, while the drying cost of mixed flow circulating vertical dryer and mixed flow static chamber dryer using coal and biomass fuel hot blast stove is 0.08–0.09 yuan/kg. Sixth, each dryer has its own shortcomings. In the process of dust drying, the cross flow (mixed flow) circulating vertical dryer will have inconvenient access to materials, while the static horizontal dryer will have uneven drying and dehydration. Relatively speaking, mixed flow static chamber dryer has fewer disadvantages.
2.1.3. Variation of Seed Temperature and Water Content in Male Sterile Lines by Various Drying Machines
During the drying process, the moisture and seed temperature of seeds dried by various drying machines were measured every 1 to 2 h, and the variation rules of moisture and seed temperature of sterile seeds dried by various drying machines were analyzed.
(1) Circulation Vertical Dryer. Cross flow circulating vertical dryer and mixed flow circulating vertical dryer are collectively referred to as circulating vertical dryer. In the drying process, seed moisture and seed temperature had the same change rule. Seed moisture decreased with drying time, while seed temperature increased with drying time. When the seed water content is above 25%, the seed temperature is 27–32°C. When the seed water was 25%∼15%, the seed temperature was 33∼37°C. When the seed moisture was 15%∼11%, the seed temperature was 38∼40°C. In the early drying stage, the seed temperature was below 35°C when the seed water content was more than 20%.
(2) Static Horizontal Dryer. When static box dryer, self-built drying bed or drying room, improved smoke room and static horizontal dryer dry seeds, the seeds are in a static state. The hot air of the combustion furnace always carries away the water from the bottom up through the seed layer. The water content of the seed has the same law with the change of seed temperature, but the water content and the change of seed temperature are different in different seed layers. The lower seeds heated up quickly. After drying for 2∼3 h, the seed temperature rose to 35°C and then gradually rose to 39°C. The temperature of the upper seeds rose slowly, and the seed temperature rose above 35°C after drying for 14 h. The lower seeds dehydrated quickly, with an average rate of 0.88%/h. The upper layer seeds were dehydrated slowly, with an average rate of 0.70%/h. The moisture difference between the upper and lower layers was about 10 percentage points during 6–10 h drying. At the end of drying, the water content of the upper seeds is about 13% and that of the lower seeds is about 10%.
(3) Mixed Flow Static Chamber Dryer. Mixed flow static chamber dryer is a new type of grain dryer which integrates mixed flow air and static continuous drying. The seed is basically at rest, with hot air passing through it from multiple directions. During the whole drying period, the seeds only need to circulate the layer 3∼5 times, each time about 20 min, and the actual circulation of all seeds is only 1∼2 times. It was observed that the high hot air of (55 ± 2) °C was used for continuous drying. About 3 h after the drying, the grain temperature reached above 40°C and about 45°C after 6 h. The seed water decreased slowly in the first 4–6 h, dehydrated rapidly in the first 6–10 h, with an average dehydration rate of more than 2.0%/h, and was kept about 0.5%/h after that. In the drying process, the seed temperature can be rapidly raised to 40∼45°C and maintained until the end of drying, which is the key and innovation of the high dehydration rate of this kind of dryer, and it is difficult to achieve other grain dryers.
2.2. Key Technologies of Mechanical Drying of Rice Seeds
2.2.1. Objective of Mechanical Drying of Sterile Line Seeds
Years of practice showed that mechanical drying of sterile seeds should achieve the following five objectives.(1)Seed safety: the germination rate and vigor of mechanically dried seeds could reach or exceed the normal drying level.(2)Fast drying: the drying dehydration rate reached (0.8 ± 0.1) %/h; that is, the seed water decreased from 30.0% to 11.5% at 24 h.(3)Low breakage rate: the damage rate of mechanically dried seeds was similar to that of normal dried seeds.(4)No mixture: the drying machine is easy to clean, can be completely cleaned, and will not cause mechanical confusion.(5)Low pollution: drying machinery itself or through supporting facilities produced low noise, less dust, and basically no mechanical noise and dust pollution to the surrounding environment.
2.2.2. The Key Technology of Mechanical Drying to Maintain High Vigor of Seeds
The testing and drying practice of various grain drying machines for many years indicated that mixed flow static chamber dryer and mixed flow circulating vertical dryer should be preferred for mechanical drying sterile seeds. Secondary static horizontal dryer or modified smoke room or self-built drying bed (room) chooses low temperature continuous drying mode of drying machinery.
It is advisable to improve the mixed circulation vertical dryer. For example, increase the drying layer and install frequency converter, by using self-flow groove and self-flow pipe; improve the bottom structure of the hoist, in order to improve the drying dehydration rate and reduce damage by convenient cleaning.
Static horizontal dryer and other series need supporting grain suction device, in order to facilitate the drying of seeds. In order to increase the rate of drying and dehydration, the ventilation system of the flue-curing room and self-made drying room should be improved.
Air heat pump is the first choice for drying heat source; biomass combustion furnace is the second choice. The hot air temperature provided by the air heat pump is stable and controllable, the relative humidity is low, and it is completely pollution-free and the use cost is low. Although the hot air provided by biomass as fuel has poor temperature stability and requires more labor, it has little pollution to the environment and low cost of purchase and use.
The suitable capacity of the vertical and chamber drying machines is 10∼20 t, which can achieve the effect of fast feed drying on a single machine.(1)Suitable harvest time: because sterile seeds are outcrossing, they have some specificity, such as being easy to germinate on ear, high enzyme activity, and being easy to fission, and need to be harvested in time. If the weather forecast shows harvest period rain, we need to grab dry harvest.(2)Rapid transit: due to the high water content (about 30%) and impurities of sterile seeds that have just been harvested and threshed, there will be fever, bad seeds, and mildew. The threshed seeds need to be quickly transferred to the drying site and quickly loaded into the dryer for drying.(3)Primary selection of seed materials: due to sterile seed production mother seed setting rate of 40%, just harvested threshing seeds hollow chaff grain, there are straw, hairy grass, and other impurities. These impurities account for about 20% of the volume. If it is harvested in rainy days, there are many impurities, which is easy to block the drying machine and reduce the drying efficiency. Therefore, the drying site should be equipped with a seed primary separator, and the seeds enter the dryer after the primary selection. At this time, most empty particles, straw, hairy grass, etc. have been removed.
Two kinds of drying processes, one-time drying method and two-stage drying method, were put forward. One-time drying method means drying the seed material to the specified moisture at one time. A method is used when the weather is normal and clear to ensure that the seeds are harvested and dried during a suitable harvest period. Two-stage drying is a method of drying the seed material in two stages to specified moisture. It is used when the seeds need to be safely harvested from the field in a short period of time due to rain forecast during the suitable harvest period or due to insufficient drying capacity of drying machinery.
The first stage of the two-stage drying method is to dry the raw material seeds recovered from the field to the water content of 16.5% below; then the seeds are removed from the dryer bag, stored in dry and ventilated room temperature warehouse, and can have short-term storage for 3–5 days. The dryer then dries the other seeds recovered from the field. After all the seeds in the field are dried in the first stage, all the seeds dried in the first stage are introduced into the dryer for the second stage drying, and the water content of the seeds is dried to 11.5%.(1)Setting of air temperature for drying machinery: mixed circulation vertical dryer is (43 ± 2) °C. Mixed flow static chamber dryer is (55 ± 5) °C. Static horizontal (box) dryer is (40 ± 2) °C.(2)Regularly monitor seed moisture and seed temperature. For mixed flow circulation vertical dryer, when the seed moisture >25%, the seed temperature control is about 30°C. When the seed moisture was 25%∼15%, the seed temperature was controlled at 35°C. When the seed moisture was less than 15%, the seed temperature was controlled at 40°C. For static horizontal (box) dryer, in the lower layer of drying bed, when the seed moisture >20%, the seed temperature control is about 35°C. When the seed moisture is less than 15%, the seed temperature is controlled at 40°C. For mixed flow static chamber dryer, the seed temperature should be raised to (43 ± 2) °C as soon as possible and kept until the end of drying.(3)Discharging: when the seed moisture decreased to 12.5%, the water was measured every 0.5 to 1.0 h. When the seed moisture decreased to 11.5%, the drying was completed and the material was discharged.
3. Methodology
3.1. Intelligent Baking Room
This paper chooses the cross flow seed intelligent baking room, which is the first batch of advanced baking introduced in China. The advantages of the intelligent baking room are low production cost, good adaptability, simple structure, and convenient installation and maintenance. However, its disadvantage is that the drying is not uniform, making part of the seed moisture content unable to meet the relevant national storage standards.
In this paper, sterile rice seeds planted in northeast China were used as drying objects. The heating temperature of sterile rice seeds should be lower than 55°C. In order to ensure the quality of sterile rice seeds after drying, reduce the burst rate and scorched grain phenomenon; it is necessary to adopt a lower medium temperature. Usually, the blast temperature is less than 60°C, so that the actual contact temperature of sterile seeds is between 35°C and 38°C.
The change of natural gas flow rate is the main factor that causes the temperature change of intelligent grilling room. The temperature remains stable and the change of natural gas flow rate is very small when the intelligent grilling room operates under normal working conditions. Therefore, the mathematical model of the intelligent baking room can be expressed as the second-order transfer function form containing the lag link, as shown in .where Z is gain and N is time constant. τ is the delay time; s is the natural gas flow rate input to the model, Nm3/h. The output is the temperature of hot blast stove, °C.
Conventional PID controller is used in the temperature control loop of hot blast stove in production site. The natural gas flow easily disturbed by other factors is selected as the internal loop, so that the interference can be effectively suppressed, and the cascade controller of hot blast stove temperature is constructed. Air fuel ratio coefficient Z is introduced. Z is the ratio of air and natural gas flow, and the set value of the air flow controller is the actual natural gas flow multiplied by Z. After setting the parameters of the air flow controller, the air fuel ratio can maintain a reasonable ratio relationship because of the rapid dynamic response of the air flow, in order to ensure the temperature stability, so that the grain drying quality can reach the ideal expectation. The temperature control structure block diagram of hot blast stove is shown in Figure 1.
3.2. Controller Design
3.2.1. Smith Predictive Compensation Control
Smith predicted that the principle of compensation control is through the reverse parallel of a compensation link on the controller. By isolating the pure lag link from other parts, the influence of lag can be eliminated and the stability and dynamic performance of the system can be enhanced. Its structure is shown in Figure 2. In the figure, R (n) is the system input, J (n) is the system output, and p (n) is the control quantity. D is the external interference of the system, is the Smith lagging output, and is the Smith nonlagging output.
Figure 2 shows that if the predicted model Aw(s) is completely corresponding to the controlled model Au(s), then
The closed-loop transfer function of the system is shown in
The characteristic equation of the system is shown in
It can be seen that the above equation does not contain the lag link and the system performance is improved, as shown in
Next, the sliding mode control will be designed based on the partial Au(s) without delay.
3.2.2. Design of Adaptive Integral Sliding Mode Controller Based on Smith’s Estimation
In order to eliminate chattering, the integral term is introduced into the sliding mode. However, since the action of the integral term starts from the initial moment, if the initial error becomes larger, the transient performance will be correspondingly worse. Therefore, a time variable term is added to make the system state at the initial moment on the sliding mode surface, which improves the convergence speed and robustness of the system. At the same time, adaptive algorithm is used to adjust the switching gain in real time to solve the uncertainty of the system. The system block diagram is shown in Figure 3. In the figure, is the controlled object, is the delay link, and is the transfer function of the Smith predictor. J is the output of the sliding mode controller, d is the disturbance, r is the system set point, and e = r − j is the tracking error.
It can be seen from Figure 3 that the mathematical model of the field hot blast stove is approximately a second-order transfer function including lag link, as shown in
The lag link in the system is compensated by Smith predictor, and the state equation of without lag is obtained, as shown in where p is the input and j is the output. is the system state.
Sliding mode design is shown in where, c > 0, the value of c determines the stability and existence of the sliding mode and the speed of convergence of the tracking error e.
The tracking error integral term is added to construct the integral sliding mode surface, as shown in where c1, c2 > 0 are constants. The integral sliding mode surface is used, as shown in where satisfies , so that the system state is on the sliding mode surface at the initial moment. Then,
Hence, the equivalent control term is as follows.
In order to improve the sliding mode performance, exponential approaching law is selected, and the control law is designed as shown in
The Lyapunov function is defined in
(13) is carried out under the assumption that the upper bounds of the system external disturbance and parameter perturbation uncertainty are known, but these two values are difficult to obtain in the actual process. For this reason, the conventional solution is to set the value of switching gain to be very large during parameter selection to improve the robustness of the system. However, the disadvantage of this method is that the system can generate severe jitter. In this paper, an adaptive sliding mode control method is proposed to update the switching gain in real time without serious chattering under the condition of uncertain upper bounds of external disturbance and parameter perturbation.
The adaptive parameter correction law is shown in where β > 0 is the adaptive adjustment speed.
According to (16), the adaptive algorithm adjusts the switching gain after judging the degree of state deviation from the sliding mode surface. However, since the adaptive law starts to integrate at the initial moment, cannot always be zero during this process. Therefore, as long as the sliding surface deviates, it will cause adaptive rhythmic action, resulting in infinite growth of . In view of the above phenomenon, the adaptive law is modified by setting the threshold value, as shown in where σ is a small normal value.
The Lyapunov function is defined in
Then, the system is asymptotically stable.
3.3. Hardware Design
The hardware system of the drying room consists of SIMATIC S7-200PLC, EM235 input module, EM222 output module, and SIMENS TP277 display screen, as shown in Figure 4. System main control module: select Siemens S7-200 MAIN control module CPU22XP. The input quantity is 14 digits; the output quantity is 10. It is equipped with two RS485 communication ports and can connect to a maximum of seven expansion ports. Input module: the temperature sensor passes the temperature information to the master control template. The control system uses EM235, which is a 4-channel analog input, and the output signal is voltage or current signal. Output module: output instructions through the output module to output digital signal control heating dryer and power regulator, circulating fan. EM222 is selected, which is 8-way relay output type. Man-machine interface equipment: SIMENS TP277 display screen is selected and connected with S7-200 PLC to realize convenient parameter setting for operators. Actuator: power regulator, dryer, circulating heat fan. The temperature acquisition is completed by entering and leaving the master control template. After analysis, the command is sent through the output module to enable the dryer to start heating. As the difference between the temperature in the drying room and the set temperature decreases, the power regulator begins to adjust the heating power and reduce energy consumption. When the temperature reached the set temperature, the circulating hot air blower started to work, and the sterile rice seeds were dried by hot air circulation.
4. Result Analysis and Discussion
4.1. Drying Rate of Sterile Line Seeds
The analysis results of seed dehydration rate of different sterile line seed varieties dried by dryer are listed in Table 2.
As can be seen from Table 2, the drying and dehydration rates of the three kinds of dryer drying are 0.69, 0.78 and 0.93, respectively, with an average of 0.8. The results showed that the drying rate of different varieties of seeds was different. The dehydration of Xiangling 628S was the fastest, while that of Longke 638S was slower.
4.2. Germination Rate and Vigor of Sterile Line Seeds by Drying
The germination potential and germination rate of male sterile line seed samples dried by dryer were detected, and the germination index and vigor index were calculated. The results of comparing the same varieties of seeds dried in a dryer with those dried in natural air are shown in Table 3.
By analyzing Table 3, it can be seen that the germination rate, germination potential, germination index, and vitality index of the seeds of the three varieties dried by dryer are slightly different from those of the seeds dried by air. The germination rate of dried seeds was 3∼9 percentage points lower than that of air dried seeds, but they were more than 81%. Xiangling 628S accounted for higher germination potential, germination index, and vigor index of machine dried seeds than those of air dried seeds. The germination potential, germination index, and vigor index of Longke 638S and Hejing 4155S were not significantly different from those of air dried seeds. The results showed that there was little difference between the seed vigor after drying and that of air dried, which proved that the dryer could be used to dry different kinds of sterile seeds. The practice of seed drying on a large scale for many years also proves that drying of sterile seeds by dryer is safe and reliable.
4.3. Drying Characteristics of Sterile Line Seeds
The temperature and moisture of seeds were observed every 2 hours at 6 points in the upper, middle, and lower layers of the recumbent drying box. The analysis results of seed temperature and water content in the upper, middle, and lower layers are shown in Figures 5 and 6.
Figure 5 shows that the moisture content and change of seeds in different layers of the drying oven are different, and the lower layer is dehydrated quickly, while the upper layer is dehydrated slowly. The whole drying process can be divided into three stages: slow dehydration stage, dehydration rate is about 0.25 and the duration is 2∼4 h, rapid dehydration, dehydration rate is about 1.0 and the duration is 14∼16 h, and gentle dehydration period, the dehydration rate is about 0.3 and the duration is 4–6 h. The moisture in the upper layer is about 4 percentage points higher than that in the lower layer. It indicates that the dehydration of different layers above and below the drying box of the dryer is not uniform.
Figure 6 shows that the temperature of seeds in the upper and lower layers in the drying box changes rapidly in the lower layer, slowly in the upper layer and in the middle layer, but all of them show low seed temperature in the early stage. When the seed moisture decreased to 17%∼18%, the seed temperature gradually increased, closely to the drying set temperature. At the end of drying, the middle seed temperature was 39.5°C, which was 0.5°C different from the drying temperature of 40°C.
The moisture content of seeds in the drying oven is divided into three levels, which are 10.5%, 12%, and 14%, respectively. Then, the seeds of the upper, middle, and lower layers were sampled at 6 points, and the seed vigor was tested. The test results were shown in Table 4.
It can be seen from Table 4 that the drying dehydration rate and seed vigor of different layers above and below the drying box of the dryer are different. The seeds in the upper layer have lower dehydration rate and higher seed vigor, while those in the middle and lower layers have higher dehydration rate and lower seed vigor.
5. Conclusion
Due to the large seed production area of rice, the seed production time is relatively consistent, and the harvest time is relatively concentrated. It is from late April to early and mid-June every year, which is in the rainy season. Due to the lack of supporting seed drying places, a significant portion of seeds are scrapped by rain every year, causing significant losses. Rice seed drying is an important link in the process of rice breeding. In order to ensure the high germination rate and production efficiency of rice seeds after drying, this paper makes a preliminary study on the dehydration and drying technology of rice male sterile line seeds in intelligent baking room and proposes an adaptive whole process integral sliding mode control based on Smith prediction. Smith predictor is used to compensate the lag link in the system to ensure the effect of sliding mode control. An adaptive full integration sliding mode surface is proposed to eliminate steady-state error and chattering. The application of this algorithm not only makes the sterile line seeds achieve the expected drying effect, but also puts forward a good strategy for the temperature control in the drying process of rice. The results of this study will provide some theoretical and practical guidance for promoting the development of mechanical drying technology of rice seeds and the safe production of rice seeds. In this experiment, there is no significant difference in the appearance of rice seeds among the treatments, but there are great differences in the germination rate. Whether it is due to the influence of varieties, maturity and other factors or the influence on the physiological and biochemical mechanism of seeds during drying needs to be further studied.
Data Availability
The labeled dataset used to support the findings of this study are available from the author upon request.
Conflicts of Interest
The author declares that there are no conflicts of interest.
Acknowledgments
This study was supported by Xichang University.